An apparatus of this kind is described and illustrated in European Patent Application No. 0 102 217: see in particular FIGS. 5A and 5B.
In the known design two couplings are provided for fastening the testing device to the connection side of the apparatus that receives it, these couplings being half-way up the testing device, i.e. to the right and left of the plug connection that is arranged approximately in the middle. The moveable parts of the couplings are arranged on the testing device and comprise cylinders with peripheral grooves in the form of screw threads, rotatable about axes at right angles to the connection side. The fixed coupling members, in the form of lugs arranged on the apparatus, are held in the peripheral grooves. Coupling is effected by rotating the cylinder around the above-mentioned axis, whereby the testing device is pressed against the connection side of the apparatus because of the arrangement of the peripheral grooves on the inclined plane principle. The cylinders are connected together by an endless driving cord near the periphery of the testing device, engaged by a common drive motor for rotation of the cylinders.
The known design gives a relatively loose coupling of the testing device, since the abutment faces (cf. FIG. 5C) that are formed by the end faces of attachments that receive centering pins are relatively small. Enlargement of the abutment faces would necessarily lead to higher costs of construction.